Superheat control for a refrigerant vapor compression system

ABSTRACT

A refrigerant vapor compression system includes a compressor, an expansion valve, a compressor speed sensor operatively connected to the compressor, an ambient temperature sensor, and a controller operatively coupled to the expansion valve, compressor speed sensor and ambient temperature sensor. The controller including a superheat control that is configured and disposed to selectively activate the expansion valve to establish a desired superheat value based on a speed of the compressor as sensed by the compressor speed sensor and ambient temperature as sensed by the ambient temperature sensor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage Application of PCT Application No.PCT/US11/048948 dated Aug. 24, 2011, the disclosure of which isincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

Exemplary embodiments pertain to the art of refrigerant vaporcompression systems and, more particularly to a system for stabilizingsuperheat based on ambient temperature and compressor speed to provideenhanced operation.

Superheat, or an amount of heat added to a refrigerant vapor after achange in state is a measure of system performance of a refrigerantvapor compression system. More specifically, super heat is a performanceindicator for how well an evaporator portion of the refrigerant vaporcompression system is performing Too much superheat indicates that theevaporator portion is not receiving enough refrigerant. Conversely, toolittle superheat indicates that the evaporator is being flooded orover-fed with refrigerant. The amount of refrigerant fed to theevaporator is controlled by an expansion valve. The expansion valve isopened/closed to control refrigerant flow to the evaporator based uponsteady state control limits. That is, at present, superheat values arefixed targets based on specific ambient temperatures and pre-determinedoperating conditions. Such control limits to not provide for enhancedperformance during transient periods such as during start-up, defrostentry and exit, or compressor speed changes.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed is a refrigerant vapor compression system including acompressor, an expansion valve, a compressor speed sensor operativelyconnected to the compressor, an ambient temperature sensor, and acontroller operatively coupled to the expansion valve, compressor speedsensor and ambient temperature sensor. The controller includes superheatcontrol that is configured and disposed to selectively activate theexpansion valve to establish a desired superheat value based on a speedof the compressor as sensed by the compressor speed sensor and ambienttemperature as sensed by the ambient temperature sensor.

Also disclosed is a method of controlling superheat in a refrigerantvapor compression system. The method includes sensing ambienttemperature, detecting operational speed of a compressor of therefrigerant vapor compression system, and establishing a desiredevaporator superheat value based on ambient temperature and operationalspeed of the compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

The following descriptions should not be considered limiting in any way.With reference to the accompanying drawings, like elements are numberedalike:

FIG. 1 is a schematic representation of a refrigerant vapor compressionsystem shown operating in a heating mode including a superheat controlin accordance with an exemplary embodiment; and

FIG. 2 is a flow chart illustrating a method of controlling superheat inaccordance with the exemplary embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A detailed description of one or more embodiments of the disclosedapparatus and method are presented herein by way of exemplification andnot limitation with reference to the Figures.

With reference to FIG. 1, a refrigerant vapor compression airconditioning system in accordance with an exemplary embodiment isindicated generally at 2. Refrigerant vapor compression system 2includes a compressor 4, an accumulator 6, and a condenser assembly 10.In accordance with an aspect of the exemplary embodiment, compressor 4takes the form of a variable speed compressor. Condenser assembly 10includes a condenser coil 12 and a condenser fan 14. Condenser coil 12and condenser fan 14 define an indoor system 16 of refrigerant vaporcompression system 2. Refrigerant vapor compression system 2 alsoincludes a heating expansion valve 20 and an evaporator assembly 24. Ina manner similar to that described above, evaporator assembly 24includes an evaporator coil 27 and an evaporator fan 30. Evaporatorassembly 24 also includes a distributor (not shown) to divide therefrigerant flow into multiple circuits through condenser coil 12.Compressor 4, accumulator 6, heating expansion valve 20 and evaporatorassembly 24 collectively define an outdoor system 33 of refrigerantvapor compression system 2. Compressor 4, accumulator 6, condenserassembly 10, heating expansion valve 20 and evaporator assembly 24 areconnected in a serial relationship and in refrigerant flow communicationvia refrigerant lines (not separately labeled).

In operation, refrigerant, for example R12, R22, R134a, R404A, R410A,R407C, R717, R744 or other compressible fluids pass through evaporatorcoil 27 in a heat exchange relationship with outdoor air. As the outdoorair is passed over evaporator coil 27 by evaporator fan 30. Therefrigerant absorbs heat and is transformed into a refrigerant vapor.The refrigerant vapor then passes through accumulator 6 and ontocompressor 4. Compressor 4 pressurizes the refrigerant vapor. Thepressurized refrigerant vapor is then passed into condenser coil 12.Indoor air is passed over condenser coil 12 in a heat exchangerelationship by condenser fan 14. The indoor air is heated by therefrigerant vapor and is directed into living spaces (not shown).Exchanging heat with the indoor air transforms the refrigerant vaporinto a pressurized liquid refrigerant. The pressurized liquidrefrigerant passes from condenser assembly 10 to heating expansion valve20 wherein the pressurized liquid refrigerant is transformed to a lowerpressure, lower temperature liquid refrigerant, typically to a saturatedliquid prior to entering evaporator assembly 24 where the process beginsanew. The above described process refers to a heating mode of operation.It should be understood that the flow of refrigerant can be reversed tooperate in a cooling mode. In such a case, the refrigerant bypassesexpansion valve 20 and, instead, flows through a cooling expansion valve35.

At this point it should be appreciated that expansion valve 20 andpossibly cooling expansion valve 35 is, in accordance with an exemplaryembodiment, an electronic variable orifice type expansion valve (EEV).In the heating mode, heating electronic expansion valve 20 regulates anamount of liquid refrigerant entering evaporator assembly 24 in responseto a superheat condition of the refrigerant entering compressor 4. Inorder to ensure a proper regulation of liquid refrigerant enteringevaporator assembly 24 for all temperature and all speeds of compressor4, refrigerant vapor compression system 2 includes a controller 40. Inaccordance with one aspect of the exemplary embodiment, controller 40takes the form of a proportional-integrated-derivative (PID) controllerand includes a superheat control 41, a transient operation control 42, aflooding control 43, and a memory 44. That is, instead of operatingrefrigerant vapor compression system 2 based on a single superheatvalue, the exemplary embodiment provides an adaptive superheat controlthat regulates liquid refrigerant passing into evaporator assembly 24based on a wide range of ambient temperature values and compressorspeeds.

In accordance with the exemplary embodiment, controller 40 includes amemory 42 and is operatively coupled to heating expansion valve 20,cooling expansion valve 35 and a plurality of sensors. Morespecifically, refrigerant vapor compression system 2 includes atemperature sensor 46 and a pressure sensor 49 compressor provided onthe refrigerant line at an outlet of evaporator coil 27. In addition, a4 includes a compressor speed sensor 50. At this point it should beunderstood that the particular type of sensors can vary. For example,compressor speed sensor 50 need not be an actual physical sensor. Speedcould be sensed by reading voltage and/or current passing through motorwindings of compressor 4. In addition, it should be understood thatrefrigerant vapor compression system 2 may include additionaltemperature and pressure sensors arranged to detect superheat when inthe cooling mode.

Reference will now be made to FIG. 2 in describing a superheat controlalgorithm 100 of controlling superheat in refrigerant vapor compressionsystem 2. Initially, controller 40 waits to receive a conditioning callin block 110. For purposes of the foregoing discussion, controller 40will receive a call for heating. Once the conditioning call is receivedin block 110, controller 40 dictates various operating parameters inblock 112. For example, controller 40 establishes compressor speed, fanoperation, electronic expansion valve setting and the like based onambient temperature and indoor demand (a desired temperature selectedversus the actual indoor temperature) in the call). At this point,refrigerant vapor compression system 2 is monitored to determine, inblock 114, when a steady-state or stable operation is achieved. Ifstable operation is not achieved, controller 40 adjusts the presetparameters in block 112.

Once refrigerant vapor compression system 2 is stable, superheat control41 sets a desired superheat value in block 116. The desired superheatvalue is dependent upon ambient temperature as sensed by temperaturesensor 46 and compressor speed as sensed by compressor speed sensor 50.In accordance with one aspect of the exemplary embodiment, superheatcontrol 41 refers to a look-up table stored in memory 44. The look-uptable includes a plurality of data points representing a range ofambient temperatures and range of compressor speeds each correlated todesired superheat values. Thus, for each ambient temperature andcompressor speed combination, there is listed a desired superheat value.In the event that ambient temperature and/or compressor speed fallsbetween data points, superheat control 41 interpolates the desiredsuperheat value. Once the desired superheat value is chosen, expansionvalve 20 is set to establish the desired superheat. Once established,controller 40 monitors the superheat through temperature sensor 46 andpressure sensor 49. If necessary, expansion valve 20 is adjusted tomaintain the desired superheat. With this arrangement, superheat control41 establishes an adaptive superheat value that is employed to regulateliquid refrigerant passing into evaporator based on existing conditions.In this manner, superheat control 41 enhances operation of refrigerantvapor compression system 2.

After the desired superheat is established in block 116, controller 40monitors for transient system changes in block 118. Transient systemchanges may include sudden changes in demand, sudden systeminitialization, entry into or exit from a defrost mode, and/or changesin compressor speed. If a transient system change is detected, transientoperation control 42 establishes an opening of expansion valve 20 basedon the sensed transient system change in block 118. If, for example,compressor 4 changes to a higher speed, transient operation control 42sets the desired superheat value based on steady state operation at thehigher speed and establishes the opening for the expansion valve 20.Once the a post transient position is set for expansion valve 20,controller 40 provides a waiting period, for example two minutes, toallow refrigerant vapor compression system 2 to return to stableoperation. If after the waiting period refrigerant vapor compressionsystem 2 is not stable or operation changes, controller 40 resets theposition of expansion valve 20. If the system returns to stableoperation after the waiting period superheat is controlled as discussedabove. If no transient system changes are detected, controller 40monitors for a flooding condition in evaporator assembly 24 in block130.

The partial flooding of evaporator is described as a relatively fewnumber of evaporator circuits flooding when a majority of the evaporatorcircuits are still in a superheated condition. This partial flooding isdetected by, for example, sensing a rapid change in superheat with asmall change of position of expansion valve 20. The partial floodingcondition is most often caused by frost forming on the outdoor coil inheating mode. Because frosting does not form evenly across the coil,heat ultimately is absorbed into the refrigerant circuits unevenly.Other conditions that may cause the partial flooding condition in eithercooling or heating modes include debris on the evaporator or non-uniformairflow across the evaporator. If controller 40 detects partial floodingin evaporator assembly 24, flooding control 43 slows down controllerresponse to allow refrigerant vapor compression system 2 to achieve astable operation. Flooding control 43 continues until flooding cannot bestopped by the slowed closing of expansion valve 20 in block 133, and adefrost mode is entered or refrigerant vapor compression system 2 isdeactivated in block 134.

At this point it should be understood that heating expansion valve 20and cooling expansion valve 35 can take on a variety of forms. Forexample, if the superheat control algorithm is only used in one mode,i.e., heating or cooling, the expansion device for the other mode maytake on any variety of forms including fixed orifice valves,thermostatic expansion valves (TXV), electronic expansion valves (EEV)and/or pulse-type solenoid valves.

It should also be appreciated that the exemplary embodiments enhanceoperation of a refrigerant vapor compression system by establishingsuperheat values based on actual operating conditions. That is, insteadof using a pre-programmed superheat value that is idealized for steadystate conditions, the exemplary embodiment sets the superheat valuebased on actual operating conditions. In addition, the exemplaryembodiment adjusts and refines the superheat value based on transientsystem changes and corrects for flooding conditions by adjusting theexpansion valve independently from the desired superheat value. Adaptivecontrol of the superheat enhances system efficiency, enhancesreliability and reduces energy costs. It should further be appreciatedthat while described in a heating mode, the superheat control algorithmcan also be employed in a cooling mode.

While the invention has been described with reference to an exemplaryembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe claims.

What is claimed is:
 1. A refrigerant vapor compression systemcomprising: a compressor; an expansion valve; a compressor speed sensoroperatively connected to the compressor; an ambient temperature sensor;and a controller control operatively coupled to the expansion valve,compressor speed sensor and ambient temperature sensor, the controllerincluding a superheat control configured and disposed to selectivelyactivate the expansion valve to establish a desired superheat valuebased on a speed of the compressor as sensed by the compressor speedsensor and ambient temperature as sensed by the ambient temperaturesensor.
 2. The refrigerant vapor compression system according to claim1, wherein the controller includes a memory having stored therein alook-up table, the look-up table including a plurality of superheatvalues that are correlated to ambient temperature and compressor speed.3. The refrigerant vapor compression system according to claim 2,wherein the expansion valve is a variable orifice expansion valve. 4.The refrigerant vapor compression system according to claim 1, whereinthe controller includes a transient operation control that establishesthe predicted expansion valve position to provide the desired superheatvalue following a transient system change.
 5. The refrigerant vaporcompression system according to claim 4, wherein the transient systemchange includes one of a compressor speed change, a systeminitialization, and an exit from a defrost mode.
 6. The refrigerantvapor compression system according to claim 1, wherein the controllerincludes a flooding control that selectively operates the expansionvalve based upon a sensed partial flooding condition of the evaporator.7. The refrigerant vapor compression system according to claim 6,wherein the flooding control shifts the expansion valve toward a closedposition upon detecting a partial flooding condition.
 8. The refrigerantvapor compression system according to claim 1, wherein the superheatcontrol comprises a proportional-integrated-derivative (PID) controller.9. A method of controlling superheat in a refrigerant vapor compressionsystem, the method comprising: sensing ambient temperature; detectingoperational speed of a compressor of the refrigerant vapor compressionsystem; and establishing a desired superheat value based on ambienttemperature and operational speed of the compressor.
 10. The method ofclaim 9, wherein establishing the desired superheat value comprisesselectively operating an expansion valve of the refrigerant vaporcompression system.
 11. The method of claim 10, wherein selectivelyoperating the expansion valve of the refrigerant vapor compressionsystem comprises establishing a desired orifice of the expansion valve.12. The method of claim 9, further comprising: retrieving the desiredsuperheat value from a look-up table stored in a memory, the superheatvalue being correlated to compressor speed and ambient temperature inthe look-up table.
 13. The method of claim 12, further comprising:interpolating the desired superheat value.
 14. The method of claim 9,further comprising: establishing a predicted superheat value following atransient system change.
 15. The method of claim 14, wherein thepredicted superheat value is established for a predetermined period oftime.
 16. The method of claim 14, wherein the predicted superheat valueis established following one of a compressor speed change, a systeminitialization, and one of an entry into and an exit from a defrostmode.
 17. The method of claim 14, wherein the predicted superheat valueis based upon a predicted steady state operation of the refrigerantvapor compression system following the transient operating parameterchange.
 18. The method of claim 9, further comprising: detecting apartial evaporator flooding condition; and shifting an evaporator valveof the refrigerant vapor compression system toward a closed positionbased on the detected partial evaporator flooded condition.
 19. Themethod of claim 18, wherein detecting a partial evaporator floodedcondition comprises detecting a frosted condition on at least a portionof the evaporator.
 20. The method of claim 18, further comprising:maintaining the expansion valve in the closed position until therefrigerant vapor compression system enters a defrost mode.